This application claims the priority of Korean Patent Application No. 2002-5217, filed Jan. 29, 2002, which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to track searching on an optical disc drive (ODD), and more particularly, to a method and apparatus of canceling noise from a track cross signal generated when an optical spot (pickup) moves on an optical disc in the radial direction, i.e., it traverses tracks so as to search for a desired track, a method for controlling an optical disc drive, an optical disc drive, and an optical disc reproducing apparatus.
2. Description of the Related Art
In general, track searching on an optical disc is performed by a pickup included in an optical disc drive (ODD) by one of the following ways: (i) a direct search control method by which an optical spot reaches a desired track on an optical disc while searching all tracks that the optical spot passes through when it moves from a track in the radial direction; and (ii) a coarse seek control method by which an optical spot moves directly to a predetermined point on an optical disc, i.e., near a desired track, without searching any track, and then reaches the desired track by searching adjacent tracks. With the coarse search control method, it is easy to search for a desired track, but the total access time is long. On the other hand, although the direct search control method is complicated, the total access time is short. For this reason, recently, the use of the direct search control method has spread even for long-distance search of tracks on a compact disc (CD), a digital versatile disc (DVD) drive, and so on.
FIG. 1 is a block diagram of a portion of a conventional optical disc drive. Referring to FIG. 1, the optical disc drive includes a hysteresis comparator 100, a noise removing apparatus 200 for canceling noise from a track cross signal, a controller 300, a drive 400, and a tracking actuator 500.
Here, the track cross signal is a signal read when a pickup (not shown) moves from a predetermined point on an optical disc in the radial direction and then the optical spot transverses tracks on the optical disc. The track cross signal can be used in detecting the speed and position of the optical spot (pickup). The track cross signal is binarized by the hysteresis comparator 100 and its glitch noise is removed by the noise removing apparatus 200. The hysteresis comparator 100 binarizes the track cross signal based on two reference values, i.e., upper and lower levels determined according to hysteresis set values. Then, the controller 300 generates a control signal in response to the binarized track cross signal whose noise was removed, and outputs the control signal to the drive 400. Next, the drive 400 drives the tracking actuator 500 in response to the control signal so as to move the pickup to a desired point on the optical disc.
FIGS. 2(a), 2(b) and 2(c) are timing diagrams explaining a conventional method for canceling noise from a track cross signal. Referring to FIGS. 2(a)–2(c), A denotes a track cross signal that contains noise and is detected by an optical disc. The hysteresis comparator 100 having upper and lower levels as reference levels are used to binarize the track cross signal A, thereby obtaining a track zero cross signal B. That is, it is possible to remove noise contained in the track cross signal A between the reference levels by binarizing the track cross signal A using two reference values determined according to hysteresis set values. In conclusion, the larger an interval between the two reference levels, i.e., the larger a difference between the upper and lower levels, the more the noise in a track cross signal is removed. However, reference levels, i.e., hysteresis set values, of the conventional hysteresis comparator 100 are fixed levels.
Meanwhile, the track zero cross signal B contains noise, i.e., glitch noise, which was not completely removed from the track cross signal A, by the hysteresis comparator 100. The noise removing apparatus 200 removes the glitch noise from the track zero cross signal B after one level of the track zero cross signal B changes into another level and then the changed level lasts for a predetermined time T, and outputs the result a signal C.
FIG. 3 is a waveform diagram of a track cross signal obtained by short-distance track searching at low speed, and FIG. 4 is a waveform diagram of a track cross signal obtained by long-distance track searching at high speed. As shown in FIGS. 3 and 4, the frequency of the track cross signal obtained by low-speed track searching is long ranging from several KHz to tens KHz, whereas the frequency of the track cross signal obtained by high-speed track searching is very high ranging from several KHz to hundreds KHz. A track search signal is passed through a band pass filter to remove noise from a high-frequency signal. According to the high-frequency cut-off characteristics of the band pass filter, the amplitude of a track search signal generated in a section where an optical spot (pickup) moves fast, is less than that of a track search signal in a section where the optical spot moves slowly.
To increase the rejection ratio of noise during track searching at low speed, it is preferable that a hysteresis set value be set to be high so that a difference between these two reference levels is increased. However, in the event that an offset between these two reference levels becomes excessively increased, the amplitude of the track cross signal may not fall within a predetermined range during track searching at high speed. This makes it difficult to detect the track zero cross signal B. Therefore, hysteresis set values of the conventional hysteresis comparator 100 are set such that an offset between these two reference levels is relatively small in consideration of the amplitude of the track cross signal generated when searching a certain track at high speed. However, in this case, it is possible to completely remove noise from the track cross signal during track searching at low speed.